The present invention is in a process of forming a phosphate coating on an individual part which consists of metal and the surface of which consists, at least in part, of iron or steel. The process of the invention employs an aqueous phosphating solution which is virtually free of iron(II) ions and contains zinc ions, phosphate ions, nitrate ions and autocatalytically formed nitrite ions.
It is known that workpieces which at least on part of their surfaces consist of steel or iron, such as individual parts, e.g., air filter housings, deep drawn brake parts, vehicle wheels, and other parts of automobile accessories, can be provided with zinc phosphate coatings. This is accomplished, inter alia, with phosphating solutions, which in addition to zinc ions and phosphate ions contain nitrate ions as accelerators. Such phosphating solutions are usually employed "on the iron side", i.e., while the solution is used it becomes enriched with iron which has been dissolved from the iron or steel surface by the pickling action. Chlorate is added at a controlled rate to maintain the content of iron(II) ions below the limit at which the phosphate coating begins to deteriorate. This addition keeps the iron(II) concentration in the range of from 0.05 to 1% by weight (EP-B-0 045 110).
Zinc phosphating processes of a different kind usually employ high nitrate contents and are carried out at elevated temperatures and distinguish in that the phosphating solutions in use are virtually free of iron(II) ions. In that case, autocatalytically formed nitrite transforms the iron(II) ions to iron(III) ions, which together with phosphate ions, form difficultly soluble iron phosphate (DE-A-25 40 684).
Each of the mentioned processes have in common that the formation of the phosphate coatings takes a relatively long treating period of 5 to 15 minutes.
In practice, a treatment for such long periods often can be performed only with relatively expensive equipment. For this reason processes which can be carried out within a shorter time are preferred. This is particularly desirable when the surface treatment is to be integrated in existing production sequences, which are used to treat individual parts, e.g., of the kind mentioned hereinbefore.
It is an object of the invention to provide a process for the formation of phosphate coatings on individual parts which at least in part of their surface consist of iron or steel which does not have the disadvantage that a long treating period is required and which permits phosphate coatings of high quality to be formed within a short time.